In an insulated-gate field-effect semiconductor device such as a MOSFET, it is sometimes observed that, as the drain voltage is increased beyond the saturation region, a point is reached where the drain current begins to increase abruptly. The device then enters a mode which is broadly known in the art as an avalanche breakdown mode which takes place in high-field regions between the gate and drain of the device. This kind of phenomenon is one of the most important disadvantages of known semiconductor devices such as transistors with high breakdown voltage performances.
In order to eliminate such a disadvantage of a prior-art insulated-gate field-effect semiconductor device, there have been proposed and put to use some types of MOSFET devices offering improved drain breakdown voltage characteristics. As will be discussed in more detail, some important problems are however encountered in fabricating such advanced versions of prior-art insulated-gate field-effect semiconductor devices. Such problems include achieving the low amplifier gains of the devices and establishing the complicated process steps required for the fabrication of the devices. The present invention thus contemplates provision of a further improved insulated-gate field-effect semiconductor device eliminating such problems of prior-art insulated-gate field-effect semiconductor devices and a process of fabricating such an improved insulated-gate field-effect semiconductor device.
It is, accordingly an important object of the present invention to provide an insulated-gate field-effect semiconductor device offering improved drain breakdown voltage characteristics.
It is another important object of the present invention to provide an insulated-gate field-effect semiconductor device adapted to achieve satisfactory amplifier gains in addition to the improved drain breakdown voltage characteristics.
It is still another important object of the present invention to provide an insulated-gate field-effect semiconductor device which features the ease of fabrication as well as the improved drain breakdown voltage characteristics and the satisfactory amplifier gains.